1. Field of the invention
This invention relates to an inner shield for a cathode ray tube, more particularly to an inner shield arranged with the minimized distances from an inner surface of a funnel and enable to effectively shield an earth magnetic field with vertical and horizontal magnetic field to improve degaussing efficiency.
2. Description of the prior art
Generally, electron beams emitted from an electron gun of a cathode ray tube (CRT) are affected by an earth magnetic field, so that paths of the electron beams are curved to cause the change of raster position and to deteriorate the purity and convergence characteristics of the CRT.
Accordingly, an inner shield is provided in the CRT to shield the earth magnetic field. The inner shield is arranged to the rear side of a mask frame that is suspended on a stud pin by a supporting spring, so that the inner shield extends along an inner surface of the funnel toward the electron gun.
Such a conventional inner shield is shown in FIG. 3. A V-typed notch is formed at an edge of an vertical inclined portion of the inner shield of which a front end has a rectangular shape to decrease the deflection of the electron beams caused by influence of the earth magnetic field.
In general, when the CRT is positioned in the North-South direction, the electron beams are affected with the earth magnetic field generated in the direction clockwise according to the Fleming's left-hand rule. The earth magnetic field tends to be incident to the material with high permeability at a right angle. The inner shield having a V-typed notch causes the earth magnetic field transferred from the electron gun to a screen to be deflected toward the horizontal inclined portion of the inner shield. Then the electron beams are affected in a direction counterclockwise by a vertical earth magnetic field induced from the V-typed notch.
However, when the CRT is positioned in the East-West direction, the CRT is increasingly affected by the earth magnetic field. That is, the earth magnetic field affect parallel to the horizontal portion of the inner shield, passing the V-typed notch thereof. Therefore shielding effect against the earth magnetic field is decreased, which causes the landing characteristics to be deteriorated.